Perspicacity (6)
I believe the answer is:
acumen
'perspicacity' is the definition.
(I know that acumen is a type of perspicacity)
This is all the clue.
(Other definitions for acumen that I've seen before include "Keen perception" , "brightness" , "Forming judgement" , "Mental sharpness as in business" , "Discernment - insight" .)